Polarization-maintaining (PM) fibers sustain a substantially stable state of polarization in single-mode transmission and are important in applications such as coherent communications, fiber devices and sensors and photonic switching. One type of polarization-maintaining fiber is a stress-induced birefringent optical fiber disclosed in U.S. Pat. No. 4,274,854 which issued to W. Pleibel, et al. on June 23, 1981. See also U.S. Pat. Nos. 4,515,436 and 4,529,426 which issued on May 7, 1985 and July 16, 1985, in the names of R. E. Howard, et al. and W. Pleibel, et al., respectively.
Suitable connection technology for fibers is essential to the successful use of polarization-maintaining fiber. The suitable connection technology must preserve a low loss and minimize polarization extinction ratio degradation for the optical fiber. Polarization extinction ratio is defined as the logarithym of the ratio of power leakage, which is orthogonal to the excited polarization axis, to that of the total launched power. Splice loss depends only on the transverse and longitudinal offset of the fiber cores whereas the extinction ratio depends only on the angular offset between the polarization axes of the two fibers. Accordingly, in order to obtain acceptable properties through a connection, it is important to align not only the fiber cores, but also the polarization axes.
Alignment of the polarization axes of the two fibers to be interconnected, which is very important, must be within a tolerance of about 1.degree.; otherwise the extinction ratio is degraded substantially. The task of aligning the polarization axes would be simplified if the polarization axes coincided with the geometric axes of the optical fiber cross section. Generally, however, polarization-maintaining fibers do not have geometric axes that coincide with the polarization axes. For a circular cross-section optical fiber, for example, expensive apparatus must be used for a determination of the direction of the polarization axes. However, a polarization-maintaining optical fiber having a generally rectangular transverse cross sectional configuration of an outer cladding layer, for example, is beneficial and brings the geometric axes and the polarization axes into coincidence. It facilitates the determination of the direction of the polarization axes.
In the prior art, most polarization-maintaining fiber splicing techniques have used fusion or adhesive bonding. Fusion splicing requires expensive micropositioners to align the cores in addition to rotational stages to search for the polarization axes of the fundamental mode. Furthermore, problems occur in polarization-maintaining fiber splicing when the residual stress profile of a fiber is deformed due to dopant diffusion during fusion and core deformation in stress-induced fibers. Adhesive bonding has shown some instabilities due to volume shrinkage during a curing process.
Another consideration derives from the fact that in many applications, only a relatively short length of polarization-maintaining fiber is required. Should there be misalignment of the optical fiber cores and birefringent axes, undesired effects such as excessive overall attenuation or a reduction in the signal-to-noise ratio due to polarization noise, modal noise and reflection loss may occur. In some instances, reflections can contribute to laser instability through optical feedback.
What is sought after is a passive mechanical connection system for polarization-maintaining optical fibers. The desired connection system should combine and simplify fiber end preparation, fiber core and polarization axes alignment. End preparation which is accomplished by mounting the fibers in a supporting structure and polishing provides connection components that are easier to align than bare fiberes. Also needed is alignment hardware which does not require subsequent operations for alignment retention. A mechanical connection system for polarization-maintaining fibers would eliminate the problem of stress profile deformation during fusion and volume shrinkage instabilities due to adhesive bonding. Furthermore, a passive connection system for polarization-maintaining fibers is desired. the sought-after passive system should be one which facilitates accurate alignment of polarization-maintaining optical fiber cores and axes without the need for the elaborate micropositioners that have been used in the past. As far as is known, the prior art does not include such a mechanical connection arrangement for polarization - maintaining fibers.